User units are connected to one or several networks offering products or services for sale. These products or services are subject to a conditional use, or control. For this reason, these units have a security module in charge of the identification and management of the access authorization to said networks.
These user units may take several forms, for example a pay-television decoder, a computer, a mobile phone, a palmtop, a PDA, a radio, a television, a multimedia station, or an automatic teller machine.
This security module is generally in the form of a smart card, a credit card or a SIM card, usually including a cryptographic processor. This card supplies the necessary information for identifying itself on a network by means of decryption operations using keys stored in the memory of the cryptographic processor, which is reputed be inviolable.
This security module can be in charge of exchanging confidential information with the user unit, for example when producing the key that starts the cryptographic operations associated to the identification or the payment of a product.
These security modules can be connected to several user units according to the user's needs. Such a use is allowed by the administrator of the network, and this is why the security module is generally moveable. This situation is found in the field of pay television or that of mobile phones.
Due to the value that this security module represents, particularly in electronic purse applications, the temptation is great for pirates to clone these modules.
Although the operating center is able to detect the simultaneous use of two modules having the same serial number, if this use is done at different moments it is not possible for the operating center to determine if the use is by the same module or by a clone.
A first solution is described in U.S. Pat. No. 4,672,533 and proposes to create session number that must change with each connection with the operating center. It is a previous identification to all transactions and although it answers the need to detect copies of the cards it can only function in an environment in which the on-line connection with the operating center is obligatory.